Storage Basics: Fibre Channel, Part Two

Tuesday Feb 18th 2003 by Mike Harwood

Storage Basics: Fibre Channel, Part Two

In the first part of our two-part look at Fibre Channel, we covered the reasons that Fibre Channel has become popular as well as the standards and market challenges facing Fibre Channel. In this second part we look at implementation considerations such as topologies, switches, and port types.

Fibre Channel Topologies

Just like a network topology, there are accepted layouts that are used to define how Fibre Channel is implemented. The following sections describe these three accepted layouts.

In this, the simplest of all the Fibre Channel topologies, devices are connected directly to each other. An example of such a configuration would be a server connected directly to a storage array. In a point-to-point configuration, devices are connected directly to each other, allowing the communication between devices to use the entire bandwidth of the link. The simplicity of the point-to-point configuration makes it the cheapest way to implement Fibre Channel.

Switched Fabric
In a way, the Switched Fabric topology is similar to the star network layout commonly implemented on networks that use twisted pair cabling. A central device, in this case a fibre channel switch, provides a central connectivity point for all of the devices in the fabric. This allows each node to access all other nodes that are connected to the fabric.

The configuration of a fabric topology can be either cascaded or non-cascaded. In a cascaded configuration, all switches are interconnected to form a larger mesh. This allows a device connected to one switch to access any other node connected to a switch within the fabric. In a non-cascaded configuration, the switches are not interconnected, making this design easier to implement than the cascaded; however, the performance of the fabric will significantly degrade should a switch fail. Also, the mesh design of the cascaded fabric allows for greater fault tolerance.

There are several benefits of this fabric topology, including support for a large number of nodes (over 16 million in fact), increased fault tolerance when implementing a cascaded configuration, and improved scalability.

Arbitrated Loop
The Fibre Channel Arbitrated Loop, or FC-AL as it is known, is a Fibre Channel topology that combines the advantages of the fabric topology (support for multiple devices) with the cost savings of the point-to-point topology (where there's no need for a central switch). In a FC-AL implementation, devices are connected to a central hub, which is typically cheaper than a switch. As with Ethernet networks, the hub offers no additional functionality to the connected devices beyond serving as a centralized connection point.

To establish communication between nodes in an FC-AL topology, devices must take control of the loop and then establish a point-to-point connection with the receiving device. Once this point-to-point connection has been made, data can be transferred. Once the transmissions have ended, devices on the FC-AL can once again "arbitrate" to gain control of the loop for establishing subsequent point-to-point connections.

In the FC-AL topology, there can be up to 126 nodes connected to a single link, but as you might expect, the more devices that share the loop, the lower the performance. Because of this, it is highly uncommon to attach the theoretical 126-node maximum to a single link.

Page 2: Loops, Fabric Switches, and Directors

Loops, Fabric Switches, and Directors
Fibre channel can be implemented in a variety of ways. The simplest method is to connect a Fibre channel Host Bus Adapter (HBA) to a Fibre channel device such as a storage array. This creates a very high-speed interface between two devices. Where it is necessary to connect more than two devices (which is very common), Fibre channel switches are used as connectivity points on the storage network.

Fibre channel switches are somewhat like Ethernet switches in that they provide the connectivity point that allows multiple devices to communicate. Interestingly, Fibre channel switches, like Ethernet switches, also create a single point of failure. As with regular Ethernet switches, the number of ports on Fibre channel switches varies from vendor to vendor.

When it comes to Fibre channel switches, there are three main categories of switches: loops, Fabric switches, and directors. Loop switches are perhaps the most basic of the three and are used to connect an FC-AL to the Fabric. A typical loop switch provides eight ports. Fabric switches are high-speed switches interconnected to form the Fibre channel fabric. A typical Fabric switch offers 16 to 32 ports. Finally, directors are the "Cadillacs" of Fibre channel switches, offering high bandwidth, high availability (99.999%) and high performance. Not surpisingly, directors are the most expensive of Fibre channel switches, but in environments where downtime is critical, they are invaluable. Typical directors offer 32 or more ports.

Fibre Channel Ports
Fibre Channel devices can have a variety of ports on them which perform different functions. The most common type of port is an N_Port (node port), which you will find on devices such as Fibre channel HBAs or on a storage device such as a disk array. An N_Port can only be connected to another N_Port, thereby creating a point to point link between devices, or to an F_Port (discussed next) on a fibre channel switch.

F_Ports (fabric ports) are found on the Fibre channel switch. F_Ports can only be used for connectivity to the N_Port of a Fibre channel device. In addition to the F-Port, the Fibre channel switch has an E_Port (expansion port), which performs the same basic function as the uplink port on a Ethernet switch in that it allows multiple Fibre Channel switches to be connected to each other. E_Ports can only be attached to other E_Ports.

Once you add arbitrated loop capabilities to your fibre channel network, you then get L_Ports (Loop Ports). L_Ports are used in the FC-AL topology and are part of FC-AL nodes. There are two different types of L_Ports, NL_ and FL_Ports. NL_ Ports are used to connect a node to the FC-AL topology and can only be connected to other NL_Ports or to FL_Ports. FL_Ports are used to connect the FC-AL loop to the Fibre channel switch.


Although some other technologies like iSCSI threaten to loosen its stranglehold on the storage environment, Fibre Channel is so widely implemented that it is likely to endure all newcomers to remain the dominant connectivity technology for the foreseeable future.

» See All Articles by Columnist Mike Harwood

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